What This Solves
Sizes primary roof drains, overflow (secondary) drains, and scuppers for flat and low-slope roofs using IPC table values.
Best Used When
- You are designing roof drainage for a flat or low-slope commercial building
- You need to determine the number and size of primary and overflow roof drains
- You want to size scuppers as overflow protection per building code requirements
Do NOT Use When
- You are sizing edge-mounted gutters for a sloped residential roof — Use Gutter Flow Calculator
- You need to size the downspouts or leaders that carry water from the drains to grade — Use Downspout Sizing Calculator
Key Assumptions
- Drain capacity is based on IPC Table 1106.1 for primary drains
- Overflow drains are sized per IPC Table 1106.6 with hydraulic head at the overflow level
- Rainfall intensity is uniform across the entire roof area
- Drains are unobstructed and strainers are clean
- Ponding loads from primary drain backup do not exceed structural capacity
Input Quality Notes
Use local code-required rainfall intensity. For overflow sizing, the design head is the depth of water above the overflow inlet — confirm this matches the roof structural ponding capacity.
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Calculate Roof Drain Size
For educational purposes only. Not a substitute for professional engineering judgment.
Roof Drain Sizing Overview
Roof drains collect and convey rainwater from flat or low-slope roofs to the storm drainage system. Proper sizing prevents ponding that can cause structural damage or leaks.
- Primary Drains - Main drainage from roof surface
- Overflow Drains - Secondary drains at higher elevation for redundancy
- Scuppers - Wall openings that act as overflow relief
- Siphonic Drains - High-capacity drains using siphon action
Roof Drain Capacity (IPC Table 1106.1)
| Drain Size (in) | @ 1" Head | @ 2" Head | @ 4" Head | Max Area (sf) |
|---|---|---|---|---|
| 2" | 26 gpm | 36 gpm | 52 gpm | 5,000 |
| 3" | 63 gpm | 90 gpm | 127 gpm | 12,200 |
| 4" | 121 gpm | 172 gpm | 242 gpm | 23,300 |
| 5" | 196 gpm | 278 gpm | 392 gpm | 37,700 |
| 6" | 295 gpm | 418 gpm | 590 gpm | 56,700 |
| 8" | 563 gpm | 796 gpm | 1126 gpm | 108,300 |
Flow rates in GPM at various head depths. Max area based on 1 in/hr at 4" head. Source: IPC 2021, Table 1106.1
Dome/Strainer Reduction Factors
| Dome Type | Flow Factor | Reduction |
|---|---|---|
| Standard Dome | 0.90 | 10% |
| Flat Grate | 0.85 | 15% |
| Beehive | 0.92 | 8% |
| Vandal-Proof | 0.80 | 20% |
| None | 1.00 | 0% |
Dome/strainer reduces effective flow capacity. Select "None" for theoretical maximum capacity.
About Roof Drain Design
Roof drains are essential components of flat and low-slope roof drainage systems. Proper sizing prevents excessive ponding that can cause structural damage, leaks, and premature roof failure.
Types of Roof Drains
- Primary Drains - Main drains located at low points to collect and convey rainwater
- Overflow Drains - Secondary drains set 2" above primary drains for redundancy
- Scuppers - Wall or parapet openings that provide overflow relief
- Siphonic Drains - High-capacity systems using full-pipe siphon action
IPC Requirements
The International Plumbing Code requires:
- Primary drains sized for 100% of design flow
- Overflow drains on low-slope roofs to prevent structural overload
- Overflow capacity equal to 100% of primary drain capacity
- Overflow drains set 2" above the primary drain inlet
Design Considerations
- Head Depth - Water depth over drain inlet; more head = more flow
- Dome/Strainer - Protective covers reduce effective capacity by 10-20%
- Drain Spacing - Consider maximum tributary area per drain
- Ponding Limit - Typically 4" maximum per structural requirements
- Slope - Minimum 1/8" per foot slope toward drains
Flow Calculation
The design flow rate is calculated from drainage area and rainfall intensity:
Q = (A x i) / 96.23
Where:
- Q = Flow rate (gpm)
- A = Drainage area (sf)
- i = Rainfall intensity (in/hr)
Reference & standards
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Last verified: February 2026